1. Field of the Invention
The present invention relates to a reflector incorporated in a reflective liquid crystal display device with no backlight, a method for fabricating the same, and a reflective liquid crystal display device incorporating the same.
2. Description of the Related Art
In recent years, liquid crystal display devices have been increasingly used in personal computers, TVs, word processors, video cameras, etc. There has been demands for further improvements of these appliances such as miniaturization, power-saving, cost reduction, etc. In an attempt to meet these demands, reflective liquid crystal display devices having no backlight which display images by reflecting ambient light incident thereupon have been developed.
In order to achieve bright display with a reflective liquid crystal display device having no backlight, it is important to efficiently utilize ambient light. Accordingly, a reflector incorporated in such a reflective liquid crystal display device plays a very important roll. It is thus necessary to design a reflector having the most suitable reflection characteristic and which efficiently utilizes ambient light incident upon the device from every direction, and to develop a technique for fabricating such a reflector with high accuracy and high reproducibility.
Japanese Laid-Open Patent Publication No 6-75238 discloses a reflective liquid crystal display device. The reflector incorporated in the liquid crystal display device includes convex/concave portions formed of a photosensitive resin and a film thinner than the convex/concave portions deposited over the convex/concave portions, thereby smoothing the surface of the reflector including the convex/concave portions. The reflector is used in the liquid crystal display device in combination with a guest-host mode (referred to as simply a "GH" mode hereinafter).
FIGS. 20G to 20L are top views each illustrating one of the fabrication steps for a conventional reflector 106, whereas FIGS. 20A to 20F are each cross-sectional views taken along the line A to A' in FIGS. 20G to 20L, respectively. In FIG. 20L, dashed lines represent contour lines of the reflector 106.
First, as shown in FIGS. 20A and 20G, a photosensitive resin is deposited on a glass substrate 101 so as to form a photosensitive resin layer 102a. Then, a photomask 103 including circular regions is placed over the glass substrate 101 as shown in FIGS. 20B and 20H. Then, the substrate is exposed to light and developed, thus forming cylindrical protrusions 102b on the substrate 101 as shown in FIGS. 20C and 20I. Then, the entire substrate is subjected to a heat treatment so that the protrusions 102b are adequately melted and form smooth convex portions 102c as shown in FIGS. 20D and 20J. Then, a photosensitive resin is again deposited over the entire surface of the substrate 101 including the smooth convex portions 102c so as to form a photosensitive resin layer 104 thinner than the layer 102a, thereby obtaining a surface including smooth convex/concave portions as shown in FIGS. 20E and 20K. Finally, a thin metal film is deposited on the layer 104 so as to form a reflection film 105 as shown in FIGS. 20F and 20L. A conventional reflector 106 is thus fabricated.
As shown in FIG. 20L, the reflector 106 fabricated by the conventional fabrication process includes a lot of flat regions. The optical characteristic of the reflector 106 is such that, although no interference occurs, a large portion of light incident upon the reflector 106 is reflected to a direction of regular reflection. For example, when light is incident upon the conventional reflector 106 from a direction perpendicular thereto, a large portion of the light is reflected to the direction perpendicular to the reflector 106 which corresponds to the direction of the regular reflection. Accordingly, there is only a very limited range of directions in which high-intensity reflected light is obtained. In other words, with such a conventional reflector 106, it is not possible to obtain high-intensity reflected light in a wide range of directions. Therefore, when such a reflector is used in a reflective liquid crystal display device performing multi-color display, the brightness of display would not be sufficient for practical use.
Conventional liquid crystal display devices are produced without sufficient consideration for compatibility among the liquid crystal display mode, the color filter, and the reflector. Therefore, there are undesirable situations such as where the display is bright but with low contrast; the contrast is high but with low brightness; or the brightness and the contrast are both high but with a slow response rate, a high threshold voltage, or non-uniformity in display due to inferior orientation of the liquid crystal molecules.
In order to ensure the display quality required for practical use, the application of such a conventional reflective liquid crystal display device is limited to a black-and-white display or, at the best, a 4-color display. Thus, the growing demand for multi-color displays with the growing variety of information has not been satisfied.
In order to realize a multi-color display which can be practically used, the compatibility among the reflector, the liquid crystal display mode, the color filter, and other factors need to be considered while improving the reflection characteristic of the reflector. Unlike a transmission type liquid crystal display device provided with a backlight, the reflective liquid crystal display device greatly depends upon ambient light. Thus, it is necessary to suitably design the optical characteristic and the convex/concave structure of the reflector, appropriately select a display mode from a number of display modes to best match the optical characteristic of the reflector, optimize various parameters of the display mode, and appropriately design a color filter. However, it has not been possible to realize a multi-color display even with these factors being optimized since the reflection characteristics of the reflector are not sufficient.